By virtue of excellent magnetic properties, Nd—Fe—B permanent magnets find an ever increasing range of application. The recent challenge to the environmental problem has expanded the application range of these magnets from household electric appliances to industrial equipment, electric automobiles and wind power generators. It is required to further improve the performance of Nd—Fe—B magnets.
Indexes for the performance of magnets include remanence (or residual magnetic flux density) and coercive force. An increase in the remanence of Nd—Fe—B sintered magnets can be achieved by increasing the volume factor of Nd2Fe14B compound and improving the crystal orientation. To this end, a number of modifications have been made. For increasing coercive force, there are known different approaches including grain refinement, the use of alloy compositions with greater Nd contents, and the addition of coercivity enhancing elements such as Al and Ga. The currently most common approach is to use alloy compositions having Dy or Tb substituted for part of Nd.
It is believed that the coercivity creating mechanism of Nd—Fe—B magnets is the nucleation type wherein nucleation of reverse magnetic domains at grain boundaries governs a coercive force. In general, a disorder of crystalline structure occurs at the grain boundary or interface. If a disorder of crystalline structure extends several nanometers in a depth direction near the interface of grains of Nd2Fe14B compound which is the primary phase of the magnet, then it incurs a lowering of magnetocrystalline anisotropy and facilitates formation of reverse magnetic domains, reducing a coercive force (see K. D. Durst and H. Kronmuller, “THE COERCIVE FIELD OF SINTERED AND MELT-SPUN NdFeB MAGNETS,” Journal of Magnetism and Magnetic Materials, 68 (1987), 63-75). Substituting Dy or Tb for some Nd in the Nd2Fe14B compound increases the anisotropic magnetic field of the compound phase so that the coercive force is increased. When Dy or Tb is added in an ordinary way, however, a loss of remanence is unavoidable because Dy or Tb substitution occurs not only near the interface of the primary phase, but even in the interior of the primary phase. Another problem arises in that amounts of expensive Tb and Dy must be used.
Besides, a number of attempts have been made for increasing the coercive force of Nd—Fe—B magnets. One exemplary attempt is a two-alloy method of preparing an Nd—Fe—B magnet by mixing two powdered alloys of different composition and sintering the mixture. A powder of alloy A consists of R2Fe14B primary phase wherein R is mainly Nd and Pr. And a powder of alloy B contains various additive elements including Dy, Tb, Ho, Er, Al, Ti, V, and Mo, typically Dy and Tb. Then alloys A and B are mixed together. This is followed by fine pulverization, pressing in a magnetic field, sintering, and aging treatment whereby the Nd—Fe—B magnet is prepared. The sintered magnet thus obtained produces a high coercive force while minimizing a decline of remanence because Dy or Tb is absent at the center of R2Fe14B compound primary phase grains and instead, the additive elements like Dy and Tb are localized near grain boundaries (see JP-B 5-31807 and JP-A 5-21218). In this method, however, Dy or Tb diffuses into the interior of primary phase grains during the sintering so that the layer where Dy or Tb is localized near grain boundaries has a thickness equal to or more than about 1 micrometer, which is to substantially greater than the depth where nucleation of reverse magnetic domains occurs. The results are still not fully satisfactory.
Recently, there have been developed several processes of diffusing certain elements from the surface to the interior of a R—Fe—B sintered body for improving magnet properties. In one exemplary process, a rare earth metal such as Yb, Dy, Pr or Tb, or Al or Ta is deposited on the surface of Nd—Fe—B magnet using an evaporation or sputtering technique, followed by heat treatment. See JP-A 2004-296973, JP-A 2004-304038, JP-A 2005-11973; K. T. Park, K. Hiraga and M. Sagawa, “Effect of Metal-Coating and Consecutive Heat Treatment on Coercivity of Thin Nd—Fe—B Sintered Magnets,” Proceedings of the 16th International Workshop on Rare-Earth Magnets and Their Applications, Sendai, p. 257 (2000); and K. Machida, at al., “Grain Boundary Modification of Nd—Fe—B Sintered Magnet and Magnetic Properties,” Abstracts of Spring Meeting of Japan Society of Powder and Powder Metallurgy, 2004, p. 202. Another exemplary process involves applying a powder of rare earth inorganic compound such as fluoride or oxide onto the surface of a sintered body and heat treatment as described in WO 2006/043348 A1. With these processes, the element (e.g., Dy or Tb) disposed on the sintered body surface pass through grain boundaries in the sintered body structure and diffuse into the interior of the sintered body during the heat treatment. As a consequence, Dy or Tb can be enriched in a very high concentration at grain boundaries or near grain boundaries within sintered body primary phase grains. As compared with the two-alloy method described previously, these processes produce an ideal morphology. Since the magnet properties reflect the morphology, a minimized decline of remanence and an increase of coercive force are accomplished. However, the processes utilizing evaporation or sputtering have many problems associated with units and steps when practiced on a mass scale and suffer from poor productivity.